JP7728960B2 - Multilayer biodegradable film, its manufacturing method, and environmentally friendly packaging material containing the same - Google Patents

Description

The present invention relates to a multilayer biodegradable film, a method for producing the same, and an environmentally friendly packaging material containing the same.

Plastic films commonly used for packaging applications include cellophane, polyvinyl chloride (PVC), polyethylene (PE), polypropylene (PP), nylon, and polyethylene terephthalate (PET).

However, cellophane film causes serious environmental pollution during the manufacturing process, and its production is subject to numerous restrictions, while polyvinyl chloride film emits hazardous substances such as dioxins when incinerated, and its use is subject to numerous restrictions. Polyethylene film also has poor heat resistance and mechanical properties, limiting its use to applications other than low-grade packaging. Polypropylene, nylon, polyethylene terephthalate, and other materials have relatively stable molecular structures and good mechanical properties, but if they are landfilled without special treatment after use in packaging, they accumulate in the ground without decomposing due to their chemical and biological stability, shortening the lifespan of landfill sites and causing soil contamination problems.

To compensate for the shortcomings of non-degradable plastic films, polylactic acid films, an aliphatic polyester whose resin itself is highly biodegradable, have recently been widely used. However, although this film has good mechanical properties, its inherent crystalline structure makes it less flexible, limiting its applications.

To address these issues, Patent Document 1 discloses a method for producing a film using a biodegradable aliphatic polyester other than polylactic acid alone. However, in this case, the glass transition temperature is too low, making it difficult to produce the film using the biaxial stretching method. Not only does the final film have low mechanical strength and a high thermal shrinkage rate, causing numerous problems during the processing process.

Meanwhile, Patent Document 2 discloses a method of blending polylactic acid with an aliphatic-aromatic copolyester to impart flexibility and heat sealing properties to the film. However, this method has the problem that the transparency of the final film is significantly reduced due to the low compatibility between polylactic acid and the aliphatic-aromatic copolyester and the use of a plasticizer, making it difficult to use in packaging applications where transparency is required.

In addition, when multilayer films containing polylactic acid and aliphatic-aromatic copolymer polyester are molded under specific extrusion temperature conditions, there is a large deviation in layer uniformity from position to position, making it difficult to control the film's appearance and thickness. If the deviation in uniformity from position to position is significant, layer marks and defects are likely to occur, which can adversely affect the film's physical properties. Furthermore, the difficulty in controlling thickness can also lead to problems with processability, productivity, and moldability.

Japanese Patent Application Publication No. 2006-272712 Japanese Patent Application Publication No. 2003-160202

The object of the present invention is to provide a multilayer biodegradable sheet or film in which the layer uniformity of the multilayer biodegradable film is adjusted to simultaneously improve the flexibility, transparency, noise level, and appearance properties of the film.

Another object of the present invention is to provide a method for producing a multilayer biodegradable film that can efficiently control the melt viscosity ranges of the first and second resin layers and finely adjust the thickness of each layer to adjust the uniformity (LUI) of the first resin layer of the multilayer biodegradable sheet or film to an optimal range, thereby satisfying all of the above-mentioned properties desired by the present invention.

Another object of the present invention is to provide a high-quality, environmentally friendly packaging material that is biodegradable and environmentally friendly by using the multilayer biodegradable film with adjusted uniformity.

The present invention provides a multilayer biodegradable film in which two or more different thermoplastic resin layers are alternately laminated, including a first resin layer primarily composed of a polylactic acid polymer and a second resin layer primarily composed of an aliphatic polyester resin or an aliphatic-aromatic copolymer polyester resin, and the first resin layer has a linear uniformity (LUI) of 0.2 μm or less, as expressed by the following formula 1:

<Formula 1>
In the formula 1,
When the thickness of each layer, excluding the outermost layers on both sides of the laminated first resin layer, was measured using a field emission scanning electron microscope (FE-SEM) in a cross section of the multilayer biodegradable film having a width of 500 mm and a thickness of 20 μm to 25 μm, cut in the thickness direction at a point (N) 50 mm away from one end in the width direction, a point (S) 50 mm away from the other end in the width direction, and a center point (C) in the width direction,
The t _max,N is the maximum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point N of the multilayer biodegradable film;
t _min,N is the minimum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point N of the multilayer biodegradable film;
The t _{max, S} is the maximum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point S of the multilayer biodegradable film;
tmin _,S is the minimum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point S of the multilayer biodegradable film;
The tmax _{, C} is the maximum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point C of the multilayer biodegradable film;
The t _min,C is the minimum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point C of the multilayer biodegradable film.

The present invention also provides a multilayer biodegradable sheet in which two or more different thermoplastic resin layers are alternately laminated, including a first resin layer primarily composed of a polylactic acid polymer and a second resin layer primarily composed of an aliphatic polyester resin or an aliphatic-aromatic copolymer polyester resin, and the first resin layer has a uniformity (LUI _s ) of less than 2.3 μm, as expressed by the following formula 1-1:

<Formula 1-1>
In the formula 1-1,
When the thickness of each of the laminated first resin layers was measured using a field emission scanning electron microscope (FE-SEM) at a cross section of the multilayer biodegradable sheet having a width of 650 mm and a thickness of 300 μm, the cross section was cut in the thickness direction at a point (N) 50 mm away from one end in the width direction, a point (S) 50 mm away from the other end in the width direction, and a center point (C) in the width direction,
The tmax _{, N1} , is the maximum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer, measured at point N of the multilayer biodegradable sheet;
tmin _{, N1} is the minimum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point N of the multilayer biodegradable sheet;
The tmax _{and S1} are the maximum thicknesses of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point S of the multilayer biodegradable sheet;
tmin _{, S1} is the minimum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point S of the multilayer biodegradable sheet;
The tmax _{, C1} , is the maximum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer, measured at point C of the multilayer biodegradable sheet;
The t _{min C1} is the minimum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point C of the multilayer biodegradable sheet.

The present invention also provides a method for producing a multilayer biodegradable film, which includes the steps of: preparing a first resin primarily composed of a polylactic acid polymer; and a second resin primarily composed of an aliphatic polyester resin or an aliphatic-aromatic copolymer polyester resin (Step 1); melt-extruding the first resin and the second resin, respectively, and alternately laminating the first resin layers and the second resin layers to obtain a sheet in which two or more different thermoplastic resin layers are alternately laminated (Step 2); and biaxially stretching and heat-setting the laminated sheet to obtain a multilayer biodegradable film (Step 3). The first resin layer of the multilayer biodegradable film has a linear uniformity (LUI) of 0.2 μm or less, as expressed by Equation 1.

Furthermore, the present invention provides an environmentally friendly packaging material containing the multilayer biodegradable film.

The multilayer biodegradable film according to the present invention simultaneously possesses excellent uniformity, flexibility, and transparency, and can also improve appearance characteristics and noise levels. In particular, the multilayer biodegradable film is characterized by its ability to achieve uniformity within a specific range due to minimal thickness variation from position to position.

Furthermore, the manufacturing method of the multilayer biodegradable film according to the embodiment can further improve moldability, processability, and productivity through an economical and efficient method. In particular, by controlling the melting temperatures of the first resin and the second resin and the difference between their melting temperatures, the melt viscosity ranges of the first resin layer and the second resin layer can be efficiently controlled, thereby fine-tuning the thickness of each layer at each position and adjusting the uniformity (LUI) of the first resin layer of the multilayer biodegradable film to an optimal range.

Furthermore, in addition to the above properties, the multilayer biodegradable film is also biodegradable and completely decomposes when landfilled, making it environmentally friendly. Therefore, it can be used as a packaging material in a variety of fields, providing high-quality, environmentally friendly packaging materials.

FIG. 1 is a schematic diagram of a multilayer biodegradable film according to one embodiment of the present invention. FIG. 2 is a perspective view of a multilayer biodegradable film according to another embodiment of the present invention. FIG. 3(a) is a perspective view cut along the line AA' in FIG. 2, and FIG. 3(b) is an enlarged view of the cross section. FIG. 4 is a schematic diagram illustrating a method for producing a multilayer biodegradable film according to one embodiment of the present invention.

The present invention will now be described in more detail.
The implementation examples are not limited to the contents disclosed below, and can be modified into various forms as long as the gist of the invention is not changed.

In this specification, when a part is said to "comprise" a certain component, this does not mean that it excludes other components, but that it may further include other components, unless otherwise specified.

In this specification, unless otherwise specified, the singular expressions "a," "an," and "the" are to be construed as including the singular or plural as the context requires.

In addition, all numerical ranges indicating physical properties, dimensions, reaction conditions, etc. of components described in this specification should be understood to be modified in all cases by the term "about" unless otherwise specified.

However, in this specification, terms such as "first resin layer," "second resin layer," "first," and "second" are used to describe various components, and the components are not limited by these terms. These terms are used only to distinguish one component from another.

In addition, the criteria for "one side"/"hitting surface" or "top"/"bottom" of each component will be explained based on the drawings, and these terms are only used to distinguish between components and may be used interchangeably in actual application.

In this specification, when a component is described as being formed above or below another component, this includes all cases in which the component is formed directly above or below the other component, or indirectly via yet another component.

In addition, the size of each component in the drawings may be exaggerated for illustrative purposes and does not represent the actual size. The same reference numbers refer to the same components throughout the specification.

In one embodiment, a multilayer biodegradable film is provided in which two or more different thermoplastic resin layers are alternately laminated, including a first resin layer whose main component is a polylactic acid polymer and a second resin layer whose main component is an aliphatic polyester resin or an aliphatic-aromatic copolymer polyester resin, and the first resin layer has a linear uniformity (LUI) of 0.2 μm or less, as expressed by the following formula 1:

<Formula 1>
In the formula 1,
When the thickness of each layer, excluding the outermost layers on both sides of the laminated first resin layer, was measured using a field emission scanning electron microscope (FE-SEM) in a cross section of the multilayer biodegradable film having a width of 500 mm and a thickness of 20 μm to 25 μm, cut in the thickness direction at a point (N) 50 mm away from one end in the width direction, a point (S) 50 mm away from the other end in the width direction, and a center point (C) in the width direction,
The t _max,N is the maximum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point N of the multilayer biodegradable film;
t _min,N is the minimum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point N of the multilayer biodegradable film;
The t _{max, S} is the maximum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point S of the multilayer biodegradable film;
tmin _,S is the minimum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point S of the multilayer biodegradable film;
The tmax _{, C} is the maximum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point C of the multilayer biodegradable film;
The t _min,C is the minimum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point C of the multilayer biodegradable film.

In one embodiment, two or more different thermoplastic resin layers, including a first resin layer and a second resin layer having the specific composition, are alternately laminated, and in particular, the uniformity (LUI) of the first resin layer is adjusted to 0.2 μm or less, thereby providing a multilayer biodegradable film that simultaneously possesses excellent uniformity, flexibility, and transparency, as well as improved appearance properties and noise levels.

Furthermore, in addition to the above properties, the multilayer biodegradable film is also biodegradable and completely decomposes when landfilled, making it environmentally friendly. This has the technical significance of enabling it to be used in a wider variety of fields and demonstrate excellent properties.

[Multilayer biodegradable film]

Referring to FIG. 1, a multilayer biodegradable film 100 according to an embodiment of the present invention is formed by alternately laminating two or more different thermoplastic resin layers, including a first resin layer 110 primarily composed of a polylactic acid-based polymer and a second resin layer 120 primarily composed of an aliphatic polyester-based resin or an aliphatic-aromatic copolymer polyester-based resin. The multilayer biodegradable film 100 may also include a first resin layer 110' as the outermost layer on both sides. Specifically, the outermost layer on both sides of the multilayer biodegradable film is the first resin layer 110'.

In one embodiment, two or more different thermoplastic resin layers are alternately laminated, including a first resin layer and a second resin layer having the specific main component, specifically a first resin layer having a polylactic acid polymer as its main component, and a second resin layer having an aliphatic polyester resin or an aliphatic-aromatic copolymer polyester resin as its main component, on one surface of the first resin layer. Not only can this improve flexibility and reduce noise levels, but the good interlayer compatibility between the first resin layer and the second resin layer can also improve interlayer adhesion properties, further improving moldability, processability, and productivity.
Each layer of the multilayer biodegradable film according to the embodiment will be specifically described below.

<First resin layer>
According to one embodiment, the first resin layer may contain a polylactic acid (PLA)-based polymer as a main component.

In the present invention, "major component" means that a specific component accounts for 50% or more by weight of all components, and specifically can mean 80% or more by weight, 85% or more by weight, 90% or more by weight, 95% or more by weight, 96% or more by weight, 97% or more by weight, or 98% or more by weight.

Specifically, the polylactic acid polymer may be contained in an amount of, for example, 50% by weight or more, specifically 80% by weight or more, 85% by weight or more, 90% by weight or more, 95% by weight or more, 96% by weight or more, 97% by weight or more, or 98% by weight or more, based on the total weight of the first resin layer.

Unlike petroleum-based resins, polylactic acid polymers are based on biomass, making them renewable resources. They also emit less carbon dioxide, a major contributor to global warming, during production than existing resins, and are environmentally friendly, being biodegradable by water and microorganisms when landfilled.

The polylactic acid polymer may have a weight-average molecular weight (Mw) of 100,000 to 1,000,000 g/mol, for example, 100,000 to 800,000 g/mol, 100,000 to 500,000 g/mol, or 100,000 to 300,000 g/mol. The weight-average molecular weight (Mw) may be measured by gel permeation chromatography (GPC). When the weight-average molecular weight (Mw) of the polylactic acid polymer satisfies the above range, the mechanical and optical properties of the multilayer biodegradable film can be further improved.

The polylactic acid polymer may contain L-lactic acid, D-lactic acid, D,L-lactic acid, or a combination thereof. Specifically, the polylactic acid polymer may be a random copolymer of L-lactic acid and D-lactic acid. In this case, the content of the D-lactic acid may be, for example, 1% to 5% by weight, for example, 1% to 4% by weight, for example, 1% to 3% by weight, for example, 1% to 2.5% by weight, or for example, 1% to 2% by weight, based on the total weight of the polylactic acid polymer. If the content of the D-lactic acid satisfies the above range, there may be an advantage in that the film's stretchability is improved.

The content of the L-lactic acid may be, for example, 80% to 99% by weight, for example, 83% to 99% by weight, or for example, 85% to 99% by weight, based on the total weight of the polylactic acid polymer. If the content of the L-lactic acid satisfies this range, there may be an advantage in that the heat resistance properties of the film are improved.

Meanwhile, in one embodiment, the first resin layer may contain only polylactic acid-based polymer, which is an aliphatic polyester.

In another embodiment, the first resin layer may include a resin obtained by copolymerizing a polylactic acid-based polymer with a small amount of other hydroxycarboxylic acid units. In this case, the hydroxycarboxylic acid units may include glycolic acid or 2-hydroxy-3,3-dimethylbutyric acid, and the hydroxycarboxylic acid units may be included in an amount of 5 wt% or less based on the total weight of the first resin layer.

In another embodiment, the first resin layer may include a mixed resin obtained by mixing the polylactic acid-based polymer with a small amount of vinyl acetate and vinyl laurate copolymer. In this case, the vinyl acetate and vinyl laurate copolymer may be included in an amount of 30 wt % or less based on the total weight of the first resin layer.

In another embodiment, the first resin layer may include a mixed resin obtained by mixing the polylactic acid-based polymer with a butyl acrylate-based rubber having a core-shell structure. In this case, the butyl acrylate-based rubber having a core-shell structure may be included in an amount of 10 wt % or less based on the total weight of the first resin layer.

On the other hand, according to an embodiment of the present invention, the first resin layer may contain conventional electrostatic agents, antistatic agents, antioxidants, heat stabilizers, UV blocking agents, antiblocking agents, and other inorganic lubricants, as long as the effects of the present invention are not impaired.

<Second resin layer>
According to one embodiment, the second resin layer contains an aliphatic polyester-based resin or an aliphatic-aromatic copolymer polyester-based resin as a main component.

In this case, the aliphatic polyester resin may be a resin different from that used in the first resin layer. By including a second resin layer containing an aliphatic polyester resin or an aliphatic-aromatic copolymer polyester resin as a main component in addition to the first resin layer, the multilayer biodegradable film not only further improves flexibility and reduces noise, but also maintains excellent interlayer compatibility with the first resin layer, which is primarily composed of a polylactic acid polymer, thereby maintaining excellent interlayer adhesion properties and further improving moldability, processability, and productivity.

If the multilayer biodegradable film contains only a first resin layer primarily composed of a polylactic acid polymer, the polylactic acid polymer may have good mechanical and optical properties, but may lack flexibility and be noisy, severely limiting its applications when manufactured into a multilayer biodegradable film. Furthermore, if the multilayer biodegradable film contains only a first resin layer and the first resin layer is formed by blending the polylactic acid polymer with an aliphatic polyester resin or an aliphatic-aromatic copolymer polyester resin different from the polylactic acid polymer, the transparency of the final film may be significantly reduced, limiting its use in packaging applications where transparency is required.

Specifically, the second resin layer may contain an aliphatic polyester resin or an aliphatic-aromatic copolymer polyester resin obtained by polycondensation of an acid component primarily composed of an aliphatic or aromatic dicarboxylic acid and a glycol component primarily composed of an alkylene glycol.

Specific examples of the aliphatic dicarboxylic acid include succinic acid, adipic acid, sebacic acid, glutaric acid, malonic acid, oxalic acid, azelaic acid, nonanedicarboxylic acid, and mixtures thereof.

The aromatic dicarboxylic acids include terephthalic acid, isophthalic acid, naphthalene-2,6-dicarboxylic acid, diphenylsulfone dicarboxylic acid, diphenyl ether dicarboxylic acid, diphenoxyethane dicarboxylic acid, cyclohexane dicarboxylic acid, and mixtures thereof, and may also contain other aliphatic or aromatic dicarboxylic acid components.

Specific examples of the glycol component include alkylene glycols such as ethylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, propylene glycol, neopentyl glycol, 2-methyl-1,3-propanediol, and diethylene glycol, 1,4-cyclohexanediol, 1,4-cyclohexanedimethanol, polyethylene glycol, and mixtures thereof.

In this case, to achieve high biodegradability, the content of aliphatic components in the acid component may be, for example, 30 mol% or more, for example, 40 mol% or more, for example, 45 mol% or more, or for example, 50 mol% or more. Specifically, the content of aliphatic components in the acid component may be 30 mol% to 80 mol%, 30 mol% to 70 mol%, or 40 mol% to 60 mol%. When the content of aliphatic components in the acid component satisfies the above range, it is possible to achieve the desired physical properties while further improving biodegradability.

According to an embodiment of the present invention, the aliphatic polyester-based resin or aliphatic-aromatic copolymer polyester-based resin may include, for example, one or more selected from the group consisting of polybutylene adipate terephthalate (PBAT) resin, polybutylene succinate (PBS) resin, polybutylene adipate (PBA) resin, polybutylene succinate-adipate (PBSA) resin, polybutylene succinate-terephthalate (PBST) resin, polyhydroxybutyrate-co-valerate (PHBV) resin, polycaprolactone (PCL) resin, and polybutylene succinate adipate terephthalate (PBSAT) resin. Specifically, the aliphatic polyester resin or aliphatic-aromatic copolymer polyester resin may include, for example, one or more resins selected from the group consisting of polybutylene adipate terephthalate (PBAT) resin, polybutylene succinate (PBS) resin, and polybutylene adipate (PBA) resin.

More specifically, the aliphatic polyester-based resin may include, for example, one or more resins selected from the group consisting of polybutylene succinate (PBS) resin and polybutylene adipate (PBA) resin, and may include, for example, polybutylene succinate (PBS) resin. Furthermore, the aliphatic-aromatic copolymer polyester-based resin may include, for example, one or more resins selected from the group consisting of polybutylene adipate terephthalate (PBAT) resin and polybutylene succinate terephthalate (PBST) resin, and may include, for example, polybutylene adipate terephthalate (PBAT) resin.

In one embodiment of the present invention, the second resin layer contains polybutylene adipate terephthalate (PBAT) resin, which is naturally degradable by microorganisms and is therefore environmentally friendly. It also improves mechanical properties such as breaking strength, tensile strength, elongation, optical properties, hardness, melt strength, and water resistance. In particular, it improves tensile strength and elongation and reduces Young's modulus, thereby improving flexibility while maintaining appropriate strength. In particular, the polybutylene adipate terephthalate (PBAT) resin has the advantages of superior elasticity, flexibility, and low noise levels compared to other biodegradable polyester resins, such as polylactic acid-based polymers. For these reasons, forming a second resin layer primarily composed of the polybutylene adipate terephthalate (PBAT) resin and alternately laminating it with a first resin layer primarily composed of the polylactic acid-based polymer to form a multilayer biodegradable film offers excellent flexibility and transparency while maintaining appropriate strength, while simultaneously achieving noise reduction effects. This offers the significant advantage of enabling use in a variety of biodegradable products.

Meanwhile, according to an embodiment of the present invention, the second resin layer may contain the aliphatic polyester-based resin or the aliphatic-aromatic copolymer polyester-based resin alone.

In another embodiment, the second resin layer may include a mixed resin obtained by mixing the aliphatic polyester resin or the aliphatic-aromatic copolymer polyester resin together with polyhydroxyalkanoate (PHA) units. The polyhydroxyalkanoate (PHA) unit may comprise one or more selected from the group consisting of poly[3-hydroxybutyrate] (P3-HB), poly[4-hydroxybutyrate] (P4-HB), poly[3-hydroxyvalerate] (PHV), poly[3-hydroxybutyrate]-co-poly[3-hydroxyvalerate] (PHBV), poly[3-hydroxyhexanoate] (PHC), poly[3-hydroxyheptanoate] (PHH), poly[3-hydroxyoctanoate] (PHO), poly[3-hydroxynonanoate] (PHN), poly[3-hydroxydecanoate] (PHD), poly[3-hydroxydodecanoate] (PHDD), and poly[3-hydroxytetradecanoate] (PHTD). Specifically, the polyhydroxyalkanoate (PHA) units may be contained in an amount of 30% by weight or less based on the total weight of the second resin layer.

The aliphatic polyester resin or aliphatic-aromatic copolymer polyester resin may be contained in an amount of, for example, 50% by weight or more, specifically 80% by weight or more, 85% by weight or more, 90% by weight or more, 95% by weight or more, 96% by weight or more, 97% by weight or more, or 98% by weight or more, based on the total weight of the second resin layer.

The aliphatic polyester resin or aliphatic-aromatic copolymer polyester resin may have a weight-average molecular weight (Mw) of, for example, 50,000 to 400,000 g/mol, for example, 50,000 to 300,000 g/mol, for example, 50,000 to 200,000 g/mol, or for example, 50,000 to 100,000 g/mol. The weight-average molecular weight (Mw) may be measured by gel permeation chromatography (GPC). When the weight-average molecular weight (Mw) of the aliphatic polyester resin or aliphatic-aromatic copolymer polyester resin satisfies the above range, it exhibits even better compatibility and processability with the first resin layer primarily composed of a polylactic acid polymer, thereby further improving the flexibility, transparency, and noise reduction effect of the multilayer biodegradable film while maintaining appropriate strength.

On the other hand, according to an embodiment of the present invention, the second resin layer may contain conventional electrostatic agents, antistatic agents, antioxidants, heat stabilizers, UV blocking agents, antiblocking agents, and other inorganic lubricants, as long as the effects of the present invention are not impaired.

<Third resin layer>
On the other hand, according to an embodiment of the present invention, a third resin layer mainly composed of an aliphatic polyester resin or an aliphatic-aromatic copolymer polyester resin different from that used for the second resin layer can be alternately laminated with the first resin layer and the second resin layer within a range that does not impair the effects of the present invention.

Specific examples of the aliphatic polyester resin and aliphatic-aromatic copolymer polyester resin that can be used in the third resin layer are the same as those used in the second resin layer described above.

Similarly, the third resin layer may also contain conventional electrostatic agents, antistatic agents, antioxidants, heat stabilizers, UV blocking agents, antiblocking agents, and other inorganic lubricants, provided that the effects of the present invention are not impaired.

<Corona Layer or Coating Layer>
On the other hand, in order to enhance the effect of post-processing steps within a range that does not impair the effects of the present invention, at least one surface of the film may be subjected to corona treatment to improve the processability of the film, or to an inorganic particle coating to prevent static electricity and blocking, or to a coating treatment to improve the printability with a printing layer.

In one embodiment, the multilayer biodegradable film may further include a corona layer disposed on the other side of the first resin layer. Specifically, the corona layer may be formed directly on the other side of the first resin layer.

By further including a corona layer, the multilayer biodegradable film can remove oil and other contaminants from the surface of the multilayer biodegradable film, creating a surface that is compatible with the adhesive site, thereby increasing adhesive strength. The surface can also be chemically and physically modified, further improving hydrophilicity, adhesion, printability, coating properties, deposition properties, etc.

The corona layer is formed by corona treatment of the first resin layer and may contain polar functional groups selected from the group consisting of -CO, -COOH, and -OH.

The surface tension of the first resin layer relative to the corona-treated surface may be 38 dyn/cm or more, for example, 38 to 70 dyn/cm, for example, 38 to 68 dyn/cm, or for example, 38 to 66 dyn/cm. When the surface tension of the first resin layer relative to the corona-treated surface satisfies this range, the adhesion, printability, coating properties, deposition properties, etc. of the multilayer biodegradable film may be further improved.

The thickness of the corona layer can be adjusted appropriately depending on the application and purpose of the multilayer biodegradable film, and can be, for example, 0.1 nm to 1000 nm, for example, 0.2 nm to 900 nm, or for example, 0.1 nm to 800 nm, but is not limited to these.

In another embodiment, the multilayer biodegradable film may further include a coating layer disposed on the other side of the first resin layer.

The coating layer may include a primer coating layer, which may improve antistatic performance.

The primer coating layer may be on the other side of the first resin layer, or if the multilayer biodegradable film includes a corona layer, on the other side of the first resin layer, and on the other side (lower side) of the corona layer.

Specifically, a primer coating layer may be formed by applying a primer treatment to the other surface of the first resin layer. Alternatively, a primer coating layer may be formed by applying a primer treatment to one surface (lower surface) of the corona layer disposed on the other surface of the first resin layer.

The primer coating layer may contain one or more compounds selected from the group consisting of ammonium-based compounds, phosphoric acid-based compounds, acrylic resins, and polymers such as urethane-based resins, all of which have antistatic properties.

The surface resistance of the primer coating layer may be 0.1 to 30 Ω/□, 0.2 to 28 Ω/□, 0.3 to 26 Ω/□, 0.4 to 24 Ω/□, or 1 to 20 Ω/□.

The surface resistance is measured by evaluating antistatic performance using a surface resistance meter, for example, at room temperature (22±2°C) and relative humidity (60%±10%).

The thickness of the coating layer can be adjusted appropriately depending on the application and purpose of the multilayer biodegradable film, and may be, but is not limited to, 15 nm to 50 nm, 20 nm to 45 nm, 25 nm to 40 nm, or 30 nm to 35 nm.

[Structural characteristics of multilayer biodegradable films]
Referring again to FIG. 1, the multilayer biodegradable film 100 according to an embodiment of the present invention is formed by alternating layers of two or more different thermoplastic resins, including a first resin layer 110 primarily composed of a polylactic acid-based polymer and a second resin layer 120 primarily composed of an aliphatic polyester-based resin or an aliphatic-aromatic copolymer polyester-based resin.

Furthermore, the multilayer biodegradable film 100 may include a first resin layer 110' as the outermost layer on both sides. Specifically, the outermost layer on both sides of the multilayer biodegradable film is the first resin layer 110'.

If the outermost layer of the multilayer biodegradable film is a first resin layer primarily composed of a polylactic acid polymer, it is more advantageous in terms of stretching and may be more advantageous in terms of formability, processability, and productivity. If the outermost layer of the multilayer biodegradable film contains a second resin layer primarily composed of an aliphatic polyester resin or an aliphatic-aromatic copolymer polyester resin, stretching may be difficult, casting may be difficult, and adhesion may occur easily, which may reduce formability, processability, and productivity.

According to an embodiment of the present invention, the total thickness of the outermost layers on both sides may be 5 to 40% of the total thickness of the film. Specifically, the total thickness of the outermost layers on both sides may be 10 to 40%, 15 to 40%, 20 to 40%, 25 to 40%, 30 to 40%, 30 to 38%, or 30 to 37% of the total thickness of the film.

When the total thickness of the outermost layers on both sides satisfies the above range, it is more advantageous in achieving the intended effect of the present invention, is more advantageous in stretching, and can be very advantageous in terms of formability, processability, and productivity.
The resin layer in contact with the outermost layers on both sides may be a second resin layer.

According to an embodiment of the present invention, the total number of layers of the multilayer biodegradable film can be adjusted taking into account the thickness of each individual layer of the film, but may be 5 or more layers, including the outermost layers on both sides. For example, the multilayer biodegradable film may have 5 to 500 layers, 5 to 300 layers, 5 to 250 layers, 5 to 225 layers, 7 to 400 layers, 7 to 350 layers, 7 to 300 layers, 7 to 250 layers, 7 to 220 layers, 10 to 200 layers, 10 to 150 layers, 10 to 100 layers, or 10 to 50 layers. If the multilayer biodegradable film has fewer than 5 layers, including the outermost layers on both sides, there may be a problem of reduced transparency.

Meanwhile, in a multilayer biodegradable film according to an embodiment of the present invention, the average thickness ratio of the individual layers of the first resin layer and the second resin layer excluding the outermost layers on both sides may be 1:0.5 to 2. The average thickness ratio of the individual layers of the first resin layer and the second resin layer excluding the outermost layer may be, for example, 1:0.5 to 2, for example, 1:0.5 to 1.5, or for example, 1:0.5 to 1.3. When the average thickness ratio of the individual layers of the first resin layer and the second resin layer excluding the outermost layer satisfies the above range, it may be more advantageous for adjusting the overall uniformity of the multilayer biodegradable film.

The average thickness of each individual layer of the first resin layer, excluding the outermost layers on both sides, may be, for example, 10 to 1,000 nm, for example, 50 to 800 nm, or for example, 100 to 600 nm.

The total thickness of the first resin layer excluding the outermost layers on both sides may be, for example, 3 μm to 40 μm, for example, 3 μm to 30 μm, for example, 3 μm to 20 μm, for example, 5 μm to 18 μm, or for example, 5 μm to 15 μm.

The total thickness of the first resin layer, including the outermost layers on both sides, may be, for example, 5 μm to 60 μm, for example, 5 μm to 50 μm, for example, 8 μm to 40 μm, for example, 8 μm to 30 μm, for example, 10 μm to 30 μm, or for example, 10 μm to 20 μm.

If the total thickness of the first resin layer and the average thickness of the individual layers fall within the above ranges, it may be even more advantageous in achieving the intended effects of the present invention.

The average thickness of each individual layer of the second resin layer may be, for example, 10 nm to 800 nm, for example, 50 nm to 700 nm, or for example, 100 nm to 600 nm. When the average thickness of each individual layer of the second resin layer satisfies this range, it becomes easier to adjust the uniformity, thereby further improving the appearance characteristics and mechanical properties of the film.

The total thickness of the second resin layer may be, for example, 2.5 μm to 40 μm, for example, 4 μm to 30 μm, for example, 4 μm to 20 μm, for example, 4 μm to 15 μm, or for example, 5 μm to 10 μm. If the total thickness of the second resin layer and the average thickness of the individual layers fall within the above ranges, it may be more advantageous in achieving the intended effects of the present invention.

The total thickness of the multilayer biodegradable film, including the outermost layers on both sides, can be, for example, 7.5 μm to 100 μm, for example, 9 μm to 80 μm, for example, 12 μm to 55 μm, for example, 13 μm to 50 μm, for example, 13 μm to 40 μm, for example, 15 μm to 40 μm, for example, 15 μm to 35 μm, or for example, 20 μm to 25 μm.

[Physical properties of multilayer biodegradable film]
The multilayer biodegradable film according to the embodiment has excellent uniformity, flexibility and transparency at the same time, and is characterized by low noise level.

First, in the multilayer biodegradable film, the first resin layer may have a lumen uniformity (LUI) of 0.2 μm or less, as represented by the following Equation 1:
<Formula 1>
In the formula 1,
When the thickness of each layer, excluding the outermost layers on both sides of the laminated first resin layer, was measured using a field emission scanning electron microscope (FE-SEM) in a cross section of the multilayer biodegradable film having a width of 500 mm and a thickness of 20 μm to 25 μm, cut in the thickness direction at a point (N) 50 mm away from one end in the width direction, a point (S) 50 mm away from the other end in the width direction, and a center point (C) in the width direction,
The t _max,N is the maximum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point N of the multilayer biodegradable film;
t _min,N is the minimum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point N of the multilayer biodegradable film;
The t _{max, S} is the maximum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point S of the multilayer biodegradable film;
tmin _,S is the minimum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point S of the multilayer biodegradable film;
The tmax _{, C} is the maximum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point C of the multilayer biodegradable film;
The t _min,C is the minimum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point C of the multilayer biodegradable film.

The individual layers of the first resin layer refer to each layer of the first resin layer excluding the outermost layers on both sides (110' in Figures 1 and 3). Therefore, the maximum and minimum thicknesses of the individual layers 110 of the first resin layer refer to the maximum and minimum thicknesses of the individual layers 110 of the first resin layer excluding the outermost layers 110' on both sides.

The uniformity (LUI) of the first resin layer represented by Equation 1 above can be an important physical property that determines the appearance characteristics, film thickness uniformity, and flexibility, transparency, and noise level of the multilayer biodegradable film. In particular, by adjusting the uniformity (LUI) of the first resin layer within a specific range, the uniformity of the final multilayer biodegradable film can be controlled, which can change the physical properties, appearance characteristics, and noise level of the film.

Generally, when molding a multilayer biodegradable film, if there is a large deviation in thickness from one position to another, it becomes difficult to control the appearance and thickness of the film from the perspective of uniformity. This can result in reduced flexibility and transparency of the film, increased noise, and overall poor physical properties. Furthermore, thickness deviation from one position to another can make stretching difficult, leading to problems with processability, productivity, and moldability.

Therefore, since the uniformity (LUI) of the first resin layer can be a measure of the quality of molded products such as packaging materials that contain the multilayer biodegradable film, it is very important to control the uniformity (LUI) by reducing the thickness deviation from one position to another in the first resin layer of the multilayer biodegradable film.

Referring to Figure 2, the uniformity (LUI) of the multilayer biodegradable film 100 can be measured using a field emission scanning electron microscope (FE-SEM) on the cross section of the multilayer biodegradable film 100 cut in the thickness direction (A-A') after defining the center point of the multilayer biodegradable film in the width (W) direction as "C," a point 50 mm away from one end of the width direction (W1) as "N," and a point 50 mm away from the other end of the width direction (W2) as "S."

Figures 3(a) and (b) are a perspective view (a) cut along line A-A' in Figure 2, and an enlarged view (b) of that cross section, respectively.

Referring to (b) of Figure 3, the thicknesses of the individual layers of the laminated first resin layer at each position (excluding the thickness of the outermost layer 110' on both sides), i.e., the thickness at point N ( _tN1 , _tN2 , _tN3 ...), the thickness at point C ( _tC1 , _tC2 , _tC3 ...), and the thickness at point S ( _tS1 , _tS2 , _tS3 ...), are measured, and the maximum thicknesses tmax _{, N} , tmax _{, C} , _{tmax, S and the minimum thicknesses tmin, N, tmin , C} , tmin _{, S} are calculated from the thicknesses of the individual layers of the laminated first resin layer at each position, and the uniformity (LUI) expressed by Equation 1 can be measured using these values. That is, the uniformity (LUI) of the first resin layer may be a value obtained by calculating the thickness of each individual layer of the stacked first resin layer, excluding the outermost layers on both sides, subtracting the minimum value from the maximum value of the thickness at each position, and dividing the result by 3.

The uniformity (LUI) of the first resin layer may be lower as the positional thickness deviation of the first resin layer is lower, and may be higher as the positional thickness deviation is higher.

In the multilayer biodegradable film, the uniformity (LUI) of the first resin layer may be, for example, 0.2 μm or less, for example, 0.15 μm or less, for example, 0.12 μm or less, for example, 0.1 μm or less, or for example, 0.08 μm or less.

In the multilayer biodegradable film, if the uniformity (LUI) of the first resin layer satisfies the above range, the positional deviation in uniformity of the multilayer biodegradable film can be reduced, thereby improving the appearance characteristics of the film, further improving flexibility and transparency, and improving noise characteristics. Furthermore, if the uniformity (LUI) of the first resin layer satisfies the above range, thickness adjustment and thickness deviation adjustment can be easily performed, further improving processability, thereby further improving processability, productivity, and moldability. If the uniformity (LUI) of the first resin layer exceeds the above range, problems may occur with the appearance characteristics of the film, positional uniformity may be poor, and marks and defects may occur in each layer, resulting in reduced physical properties at that position.

In one embodiment of the present invention, when the multilayer biodegradable film has 5 to 30 layers and a film thickness of 20 to 25 μm, the uniformity (LUI) of the first resin layer may be, for example, 0.1 μm or less, for example, 0.09 μm or less, or for example, 0.08 μm or less.

In another embodiment of the present invention, when the multilayer biodegradable film has more than 30 layers but less than 50 layers and the film thickness is 20 to 25 μm, the uniformity (LUI) of the first resin layer may be, for example, 0.15 μm or less, for example, 0.12 μm or less, or for example, 0.10 μm or less.

In another embodiment of the present invention, when the multilayer biodegradable film has 50 or more layers and a film thickness of 20 to 25 μm, the uniformity (LUI) of the first resin layer may be, for example, 0.2 μm or less, for example, 0.15 μm or less, or for example, 0.13 μm or less.

The smaller the uniformity (LUI) of the first resin layer within the above range, the more advantageous it may be in achieving the objectives of this embodiment of the present invention.

In addition, the multilayer biodegradable film according to an embodiment of the present invention may have a uniformity difference (Δt _N,S ) between both ends in the width direction of the multilayer biodegradable film, represented by the following formula 2, of 0.06 μm or less.

<Formula 2>
In the formula 2,
The t _{max,N 1} , t _{min,N 2} , t _{max,S 1} , and t _min,S are as defined above.

Specifically, the difference in uniformity (Δt _N,S ) between both ends of the multilayer biodegradable film in the width direction may be, for example, 0.05 μm or less, for example, 0.03 μm or less, for example, 0.027 μm or less, for example, 0.025 μm or less, for example, 0.02 μm or less, or for example, 0.015 μm or less. The closer the difference in uniformity (Δt _N,S ) between both ends of the multilayer biodegradable film in the width direction is to 0, the more advantageous it is. In this case, the thickness deviation between both ends of the film is reduced, improving the appearance characteristics of the film, further improving flexibility and transparency, and improving noise characteristics. If the difference in uniformity (Δt _N,S ) between both ends of the multilayer biodegradable film in the width direction is outside the above range, problems may occur in the appearance characteristics of the film, such as poor uniformity and the occurrence of marks or defects in each layer, which may result in reduced physical properties at that location.

According to one implementation of the present invention, for N points, the difference between the t _max,N and t _min,N may be, for example, 0.15 μm or less, such as 0.13 μm or less, such as 0.12 μm or less, such as 0.1 μm or less, such as 0.08 μm or less, or such as 0.07 μm or less.

For point S, the difference between said t _{max, S} and t _{min, S} may be, for example, 0.15 μm or less, such as 0.12 μm or less, such as 0.1 μm or less, or such as less than 0.1 μm.

For point C, the difference between the t _{max, C} and t _{min, C} may be, for example, 0.2 μm or less, for example, 0.15 μm or less, for example, 0.13 μm or less, for example, 0.12 μm or less, or for example, 0.1 μm or less.

When at least one of the difference between tmax _{, N} and tmin _{, N} , the difference between tmax _{, S} and tmin _{, S} , and the difference between tmax _{, C} and tmin _{, C} of the multilayer biodegradable film satisfies the above range, the thickness deviation of the film can be reduced, improving the appearance properties of the film, further improving flexibility and transparency, and improving noise characteristics.

Meanwhile, the multilayer biodegradable film may have a flexibility noise composite (FNC) of 20 or less, as expressed by the following formula 3:

<Formula 3>
In the formula 3,
The YM and N _AVG are unit-free values measured on a test piece of the multilayer biodegradable film,
The YM is Young's modulus (kgf/mm2), which is a linear gradient value from the measurement start point to the point where an elongation rate of 3% is reached, after preparing a multilayer biodegradable film specimen according to ASTM D882, cutting it into a length of 150 mm and a width of 15 mm, and attaching it so that the gap between chucks is 50 mm and testing it at a pulling speed of ²⁰⁰ mm/min.
The N _AVG is the average noise intensity (dB) calculated by measuring five times the noise intensity while a multilayer biodegradable film cut to A4 size of 210 mm x 297 mm was placed 30 cm away from a digital noise analyzer in a polycarbonate box of 650 (W) mm x 450 (D) mm x 500 (H) mm, and the film was gripped at both ends with a jig and repeatedly twisted back and forth at a speed of 30 times per minute, producing noise for at least 5 seconds.

The flexibility noise composite index (FNC) represented by Equation 3 above is the product of the Young's modulus and noise intensity of the multilayer biodegradable film divided by 1000, and is an index showing the degree of the composite properties of flexibility and noise intensity of the multilayer biodegradable film.

The flexibility noise composite (FNC) may be lower if the multilayer biodegradable film has a lower Young's modulus and/or noise index.

When the flexibility noise composite (FNC) having such characteristics satisfies the specific range or less, it can further improve the flexibility and transparency of the multilayer biodegradable film, reduce noise levels, and further improve the quality of molded products such as packaging materials that contain the multilayer biodegradable film.

The flexibility and noise composite (FNC) of the multilayer biodegradable film can be, for example, 20 or less, for example, 2 to 20, for example, 3 to 18, for example, 5 to 18, or for example, 10 to 19. When the flexibility and noise composite (FNC) satisfies this range, the flexibility and noise properties of the multilayer biodegradable film can be improved simultaneously.

Meanwhile, in Equation 3, the Young's modulus of the multilayer biodegradable film is preferably 300 kgf/mm2 or ^less , 250 kgf/mm2 or ^less , or 240 kgf/ ^mm2 or less. The Young's modulus can be measured by preparing a test piece of the multilayer biodegradable film in accordance with ASTM D882, cutting it to a length of 150 mm and a width of 15 mm, and mounting it so that the chuck spacing is 50 mm. The test piece is then subjected to an experiment using a tensile tester (Instron 5566A) at a tensile speed of 200 mm/min, and the linear gradient from the measurement start point to the point where an elongation of 3% is reached is measured as the Young's modulus.

A single-layer polylactic acid polymer film produced by a conventional method has a Young's modulus of 350 kgf/mm2 or ^more , which is extremely low in flexibility and hard, limiting its applications. When the Young's modulus of the multilayer biodegradable film satisfies 300 kgf/ ^mm2 or less, it is more advantageous to control the Flexibility Noise Composite (FNC) to 20 or less, and the desired effect can be easily achieved.

In addition, in Equation 3, the average noise intensity ( _NAVG ) of the multilayer biodegradable film was defined as the average noise intensity calculated by cutting the multilayer biodegradable film into an A4 size of 210 mm x 297 mm in a polycarbonate box of 650 (W) mm x 450 (D) mm x 500 (H) mm, positioning the multilayer biodegradable film 30 cm away from a digital noise analyzer (Cirrus Research Plc, model name: CR-162C), gripping both ends of the film with a jig, and twisting it back and forth at a speed of 30 times per minute five times while producing noise for 5 seconds or more.

It is preferable that the average noise intensity (N _AVG ) of the multilayer biodegradable film be controlled to a specific range or less in order to provide a high-quality packaging material.

Specifically, the average noise intensity (N _AVG ) of the multilayer biodegradable film may be, for example, 86 dB or less, for example, 85 dB or less, for example, 80 dB or less, or for example, 79.5 dB or less.

In particular, if the average noise intensity ( _NAVG ) of the multilayer biodegradable film is 80 dB or less, it is more advantageous in controlling the flexible noise composite (FNC) to 20 or less, and a high-quality packaging material can be provided due to the low noise intensity.

Furthermore, the multilayer biodegradable film may have a film appearance noise quality composite index (QCI) of 28 or less, as represented by the following formula 4:

<Formula 4>
In the formula 4,
The HZ, LUI, and N _AVG are unit-free values measured on the multilayer biodegradable film specimen,
HZ is the haze (%) of the multilayer biodegradable film,
The LUI and the N _AVG are as defined above.

The appearance noise quality composite index (QCI) represented by the above formula 4 is the sum of the haze, uniformity, and noise level of the multilayer biodegradable film divided by 3, and is an index that indicates the degree of the composite properties of the uniformity (LUI) of the first resin layer and the uniformity, transparency, and noise level of the final multilayer biodegradable film.

The appearance and noise quality composite index (QCI) can satisfy the above range as the haze, uniformity, and noise characteristics (noise reduction effect) of the multilayer biodegradable film are all excellent. In other words, the appearance and noise quality composite index (QCI) can be lower as the haze, uniformity, and/or noise level of the multilayer biodegradable film are lower.

When the appearance noise quality composite index (QCI) having such characteristics satisfies the aforementioned range, it can simultaneously further improve the haze and uniformity of the multilayer biodegradable film, reduce noise levels, and further improve the quality of molded products such as packaging materials that contain the multilayer biodegradable film.

The appearance noise quality composite index (QCI) of the multilayer biodegradable film may be, for example, 15 to 28, for example, 20 to 28, for example, 22 to 28, or for example, 25 to 28. When the appearance noise quality composite index (QCI) of the multilayer biodegradable film satisfies this range, flexibility, noise reduction effect, and transparency can be simultaneously improved, making it suitable for use as a packaging material in a variety of fields and further advantageous in providing high-quality, environmentally friendly packaging materials.

Furthermore, it is preferable that the haze of the multilayer biodegradable film according to the embodiment of the present invention is 10% or less, 5% or less, or 3% or less. If the haze exceeds 10%, the transparency may be insufficient, which may limit its applications.

The multilayer biodegradable film of the present invention, which is a product intended to reduce environmental impact, can have a biodegradation rate of at least 60%, preferably 80% or more, and more preferably 90% or more.

[Method for producing multilayer biodegradable film]
In one embodiment, a method for producing a multilayer biodegradable film is provided, which includes the steps of: preparing a first resin primarily composed of a polylactic acid polymer; and a second resin primarily composed of an aliphatic polyester resin or an aliphatic-aromatic copolymer polyester resin (Step 1); melt-extruding the first resin and the second resin, respectively, to alternately laminate first resin layers and second resin layers, thereby obtaining a sheet in which two or more different thermoplastic resin layers are alternately laminated (Step 2); and biaxially stretching and heat-setting the laminated sheet to obtain a multilayer biodegradable film (Step 3), wherein the first resin layer of the multilayer biodegradable film has a linear unit interface (LUI) of 0.2 μm or less, as expressed by Equation 1.

A method for producing a multilayer biodegradable film according to an embodiment of the present invention involves melt extrusion of a first resin and a second resin, each containing a specific resin as its main component, and alternating lamination of the first and second resin layers to obtain a sheet in which two or more different thermoplastic resin layers are alternately laminated. This sheet is then biaxially stretched and heat-set, thereby further improving formability, processability, and productivity. This economical and efficient method allows for the production of a multilayer biodegradable film that simultaneously possesses the excellent uniformity, flexibility, and transparency desired by the present invention, as well as improved noise levels.

In particular, the method for manufacturing the multilayer biodegradable film has the technical significance of being able to efficiently control the melt viscosity range of the first resin layer and the second resin layer by controlling the melting temperatures of the first resin and the second resin and the difference between their melting temperatures, thereby enabling fine adjustment of the thickness of each layer at each position and adjusting the lumen uniformity (LUI) of the first resin layer to an optimal range.

Referring to FIG. 4, the method for manufacturing the multilayer biodegradable film (S100) may include the step of preparing a first resin primarily composed of a polylactic acid-based polymer and a second resin primarily composed of an aliphatic polyester-based resin or an aliphatic-aromatic copolymer polyester-based resin (S110).

In one embodiment of the present invention, the first resin and second resin each have a specific melt viscosity range, thereby efficiently adjusting the melt viscosity range and melt viscosity difference of the formed first resin layer and second resin layer, thereby reducing thickness deviations at different positions in the film and controlling uniformity, thereby achieving the desired ranges of flexibility, transparency, and noise level.

Therefore, the melt viscosity of the first resin and the second resin can be a very important factor in adjusting the uniformity (LUI) of the first resin layer of the multilayer biodegradable film of the present invention.

Generally, when forming a multilayer film with five or more layers, for example, from five to several hundred layers, the thickness uniformity of the film can vary depending on the melt viscosity. In particular, differences in melt viscosity can affect the thickness uniformity and physical properties of the film. In particular, in the case of a multilayer biodegradable film made up of thin layers, the greater the number of layers, the more difficult it is to control the thickness uniformity, which can lead to problems such as greater thickness variation between layers at different positions.

In one embodiment of the present invention, the melt viscosity of the first resin and the second resin forming the first resin layer and the second resin layer is adjusted to reduce the difference in melt viscosity between the first resin layer and the second resin layer, thereby controlling the uniformity of the first resin layer to within a desired range.

Specifically, the first resin may have a melt viscosity at 210°C of, for example, 5,000 poise to 12,000 poise, for example, 6,500 poise to 11,000 poise, or for example, 8,000 poise to 10,000 poise. In this case, the melt viscosity can be measured using a rheometer (RDS).

The melt viscosity of the first resin may be the same as or similar to the melt viscosity of the first resin layer formed therefrom. Therefore, if the melt viscosity of the first resin satisfies the above range, a first resin layer having a melt viscosity in the same or similar range can be realized, and the uniformity of the first resin layer can be easily controlled to 0.2 μm or less. If the melt viscosity of the first resin is outside the above range, the thickness deviation from one position to another in the multilayer biodegradable film may increase, resulting in poor uniformity.

Meanwhile, the melting temperature (Tm) of the first resin may be, for example, 100°C to 250°C, for example, 110°C to 220°C, or for example, 130°C to 220°C.

The first resin may have a glass transition temperature (Tg) of, for example, 30°C to 80°C, for example, 40°C to 80°C, for example, 40°C to 70°C, or for example, 45°C to 65°C.

When the melting temperature (Tm) and glass transition temperature (Tg) of the first resin each fall within the above ranges, this can be even more advantageous in improving the uniformity of the first resin layer of the multilayer biodegradable film, as well as its mechanical and optical properties.

Meanwhile, the second resin may have a melt viscosity at 210°C of, for example, 4000 poise to 8000 poise, for example, 5000 poise to 7000 poise, or for example, 6000 poise to 7000 poise.

The melt viscosity of the second resin may be the same as the melt viscosity of the second resin layer formed thereby. When the melt viscosity of the second resin satisfies this range, a second resin layer having a melt viscosity in the same range can be achieved, and the difference in melt viscosity between the first resin layer and the second resin layer can be controlled to a desired specific range, which may be further advantageous in controlling the degree of uniformity (LUI) of the first resin layer of the multilayer biodegradable film.

The first resin may be the same as the resin contained in the first resin layer described above in terms of type and properties.

Furthermore, the second resin may be the same in type and characteristics as the resin contained in the second resin layer described above.

The method for manufacturing the multilayer biodegradable film (S100) may include a step (S120) of melt-extruding the first resin and the second resin, respectively, and alternately laminating the first resin layer and the second resin layer to obtain a sheet in which two or more different thermoplastic resin layers are alternately laminated.

According to an embodiment of the present invention, the melt extrusion temperatures of the first resin and the second resin can be controlled to obtain the first resin layer and the second resin layer, which can then be alternately laminated to obtain a sheet in which two or more different thermoplastic resin layers are alternately laminated. This reduces thickness deviations at different positions in the final film and controls uniformity, thereby achieving the desired ranges of flexibility, transparency, and noise level.

Specifically, the first resin and the second resin obtained in step S110 can be melt-extruded using two extruders and a multi-layer feed block in which the two layers are alternately laminated. Furthermore, the first resin layer and the second resin layer can be branched in the multi-layer feed block, and then the branched first resin layer and the second resin layer can be alternately laminated and passed through a die, and then pressed against a cooling roll cooled to approximately 10°C to 40°C to obtain an unstretched multi-layer biodegradable sheet.

In this case, the melt extrusion temperatures of the first resin and the second resin, and the difference between these melt extrusion temperatures, can be very important in order to obtain a multilayer biodegradable film whose uniformity is controlled to the specific range targeted by the present invention. Specifically, in order to obtain a multilayer biodegradable film having the specific range of uniformity, it is important to control the melt viscosities of the first resin layer and the second resin layer and the difference between these melt viscosities, which can be achieved by controlling the melt extrusion temperatures of the first resin and the second resin.

The melt extrusion temperature of the first resin and the melt extrusion temperature of the second resin may be the same or different, and the difference between the melt extrusion temperature of the first resin and the melt extrusion temperature of the second resin may be, for example, 30°C or less, for example, less than 30°C, for example, 20°C or less, for example, 15°C or less, for example, less than 15°C, for example, 10°C or less, or for example, 5°C or less. In this case, by ensuring the optimal viscosity difference range, it is possible to reduce thickness deviations at different positions in the final film and control the uniformity of the first resin layer within the above range, thereby achieving the desired ranges of flexibility, transparency, and noise level and improving appearance characteristics.

The melt extrusion temperature of the first resin may be, for example, greater than 180°C to 250°C, such as 190°C to 240°C, or such as 190°C to 230°C.

The melt extrusion temperature of the second resin may be, for example, greater than 180°C to 250°C, such as 190°C to 240°C, or such as 190°C to 230°C.

The melt viscosity and the deviation in melt viscosity of the first and second resin layers formed can be controlled by the melt extrusion temperatures of the first and second resins. If the melt extrusion temperatures of the first and second resins are outside the above range, the desired melt viscosity of the first and second resin layers cannot be achieved, resulting in large deviations in thickness at different positions in the multilayer biodegradable film, which may make it difficult to achieve the desired uniformity of the first resin layer.

In one embodiment of the present invention, the melt viscosity of the first resin layer may be greater than the melt viscosity of the second resin layer. This is advantageous because it makes it easier to control the uniformity of the first resin layer at different positions in the multilayer biodegradable film. If the melt viscosity of the second resin layer is greater than the melt viscosity of the first resin layer, it may be difficult to achieve the intended effects of the present invention, and processability, productivity, and moldability may also be reduced.

Furthermore, it may be advantageous for the melt viscosity of the first resin layer to be greater than the melt viscosity of the second resin layer, with the difference being equal to or greater than a specific melt viscosity, in order to achieve the effects achieved by the present invention.

Generally, in multilayer biodegradable films or coextruded products consisting of fewer than five layers, it is advantageous in terms of layer uniformity if the melt viscosities are similar. However, in multilayer biodegradable films consisting of five or more layers, for example, several tens of layers, particularly in structures in which the first resin layer forms the outermost layer on both sides of the film, the melt viscosity of the second resin layer is lower than that of the first resin layer, and the optimal melt viscosity difference (ΔV) between these layers is approximately 500 poises or more, for example approximately 2000 poises or more, to achieve the physical properties desired by the present invention.

If the melt viscosity of the second resin layer were similar to or greater than that of the first resin layer, the interfacial pressure between the layers would increase as the first and second resin layers pass through the very narrow slits in the multilayer block and are alternately laminated, potentially resulting in a non-uniform layer structure. By adjusting the melt extrusion temperature so that the melt viscosity of the second resin layer is lower than that of the first resin layer and a specific range of difference is achieved, optimal uniformity of the multilayer biodegradable film can be ensured.

Specifically, the interlayer melt viscosity difference (ΔV) at 210°C, as expressed by the following formula 5, can be 500 poise or more.

<Formula 5>
ΔV=V1-V2
In the formula 5,
V1 is the melt viscosity of the first resin layer,
V2 is the melt viscosity of the second resin layer.

The interlaminar viscosity difference (ΔV) at 210°C may be, for example, 700 poise or more, for example, 800 poise or more, for example, 1000 poise or more, for example, 1500 poise or more, for example, 2000 poise or more, for example, 2200 poise or more, or for example, 2500 poise or more, and may be, for example, 3500 poise or less, for example, 3200 poise or less, for example, 3000 poise or less, or for example, 2700 poise or less. Specifically, the interlaminar viscosity difference (ΔV) may be 500 to 3000 poise, or, for example, 700 to 3000 poise.

Specifically, the melt viscosity of the first resin layer at 210°C may be, for example, 7,000 poise to 12,000 poise, for example, 7,500 poise to 11,000 poise, or for example, 8,000 poise to 10,000 poise. In this case, the melt viscosity of the first resin layer may be measured using a rheometer (RDS).

The second resin layer may have a melt viscosity at 210°C of, for example, 4,000 to 8,000 poises, for example, 5,000 to 7,000 poises, or for example, 6,000 to 7,000 poises.

When the melt viscosities of the first resin layer and the second resin layer satisfy the above ranges, the uniformity of the first resin layer of the multilayer film can be controlled, which can be even more advantageous in achieving the desired effects of the present invention.

Furthermore, during the melt extrusion, sufficient quantitative transfer and plasticization can be achieved, for example, by applying a quantitative transfer device (e.g., a gear pump) within the melt transfer conduit. In this case, using two extruders and a multi-layer feed block in which two layers are alternately stacked can result in better layer formation.

Meanwhile, depending on the implementation, the method may further include a step of drying the first resin and the second resin before the melt extrusion. The drying step may be performed, for example, at 60°C to 100°C for 4 hours to 24 hours.

Meanwhile, the method for manufacturing the multilayer biodegradable film (S100) may include a step (S130) of biaxially stretching and heat-setting the laminated sheets to obtain a multilayer biodegradable film.

Specifically, the laminated sheet can be biaxially stretched. The biaxial stretching step can include, for example, preheating to 50°C to 80°C, then stretching the sheet 2x to 4x in the machine direction (MD) at 40°C to 100°C, and then stretching the sheet 3x to 6x in the transverse direction (MD) at 50°C to 150°C.

By biaxially stretching the laminated sheets in both directions, the physical properties and formability of the multilayer biodegradable film can be further improved, resulting in high-quality packaging materials.

If the multilayer biodegradable film were uniaxially stretched in only one of the machine and cross directions, the thickness deviation would be large, which could result in a decrease in the strength of the unstretched side and a decrease in thermal properties.

Furthermore, the heat setting step can be carried out at 50°C to 150°C, 70°C to 150°C, 100°C to 150°C, or 110°C to 140°C.

Meanwhile, the method for manufacturing the multilayer biodegradable film (S100) may further form a corona layer, a coating layer, or both on the other side of the first resin layer.

Specifically, a corona layer can be formed by corona treating the first resin layer.
In the corona treatment, a corona discharge occurs when a high frequency-high voltage output is applied between a discharge electrode and a treatment roll, and the corona treatment can be carried out by passing a desired surface through this process.

Specifically, the corona discharge intensity may be, for example, 3 kW to 20 kW. If the corona discharge intensity is below this range, the effect of the corona discharge treatment is minimal, whereas if the corona discharge intensity exceeds this range, excessive surface modification may cause surface damage.
The constitution and physical properties of the corona layer are as described above.

Also, a coating layer may be formed on the other surface of the first resin layer.
The coating layer may include a primer coating layer, and the primer coating layer may be formed by treating the other surface of the first resin layer with a primer composition including at least one selected from the group consisting of an ammonium-based compound, a phosphate-based compound, and polymers such as an acrylic resin and a urethane resin, thereby forming surface roughness and further improving adhesion properties.

The primer coating layer may be formed on the other side of the first resin layer, or if the multilayer biodegradable film includes a corona layer, a corona layer may be formed on the other side of the first resin layer and the primer coating layer may be formed on the other side of the corona layer.

The primer composition may also contain a curing agent component, more specific examples of which include 4,4'-diaminodiphenylmethane (DDM), aromatic diamines, and mixtures thereof. The curing agent component may be added in an amount of 0.1% to 50% by weight based on the total weight of the primer composition.

The primer treatment method can be a method commonly used in the industry, such as spraying, brushing, or rolling. Specifically, the primer composition can be sprayed onto the surface of the first resin layer using an airless spray under the following conditions: induction time of 1 to 30 minutes, spray pressure of 5 to 500 MPa, nozzle diameter of 0.46 to 0.58 mm, and spray angle of 40 to 80 degrees.

In addition, to enhance the adhesiveness of the multilayer biodegradable film, surface treatments such as plasma treatment, ultraviolet irradiation treatment, flame treatment, or soap treatment may be performed as appropriate.

Producing the multilayer biodegradable film using the manufacturing method of the embodiment is economical and efficient, and can be more effective in producing a multilayer biodegradable film with the desired structure and physical properties.

[Multilayer biodegradable sheet and manufacturing method thereof]
According to one embodiment, a multilayer biodegradable sheet can be provided in which two or more different thermoplastic resin layers are alternately laminated, including a first resin layer whose main component is a polylactic acid-based polymer and a second resin layer whose main component is an aliphatic polyester-based resin or an aliphatic-aromatic copolymer polyester-based resin, and the uniformity (LUI _s ) of the first resin layer, expressed by the following formula 1-1, is less than 2.3 μm.

<Formula 1-1>
In the formula 1-1,
When the thickness of each layer of the laminated first resin layer was measured using a field emission scanning electron microscope (FE-SEM) on a cross section of the multilayer biodegradable sheet having a width of 650 mm and a thickness of 300 μm, the cross section was cut in the thickness direction at a point (N) 50 mm away from one end in the width direction, a point (S) 50 mm away from the other end in the width direction, and a center point (C) in the width direction,
The tmax _{, N1} , is the maximum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer, measured at point N of the multilayer biodegradable sheet;
tmin _{, N1} is the minimum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point N of the multilayer biodegradable sheet;
The tmax _{and S1} are the maximum thicknesses of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point S of the multilayer biodegradable sheet;
tmin _{, S1} is the minimum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point S of the multilayer biodegradable sheet;
The tmax _{, C1} , is the maximum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer, measured at point C of the multilayer biodegradable sheet;
The t _{min C1} is the minimum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point C of the multilayer biodegradable sheet.

In the multilayer biodegradable sheet, the uniformity (LUI _s ) of the first resin layer may be, for example, 2.2 μm or less, for example, 2.0 μm or less, or for example, less than 2.0 μm.

In the multilayer biodegradable sheet, when the uniformity (LUI _s ) of the first resin layer satisfies the above range, the thickness deviation in the uniformity of the multilayer biodegradable sheet at each position can be reduced, thereby improving the appearance characteristics of the sheet, further improving transparency and flexibility, and improving noise characteristics (noise reduction effect).

The number of layers in the multilayer biodegradable sheet may be the same as the number of layers in the multilayer biodegradable film.

The total thickness of the multilayer biodegradable sheet may be, for example, 200 μm to 500 μm, for example, 250 μm to 450 μm, for example, 250 μm to 400 μm, or for example, 250 μm to 350 μm.

Meanwhile, an embodiment of the present invention can provide a method for manufacturing the multilayer biodegradable sheet.

Specifically, the method for manufacturing the multilayer biodegradable sheet may include the steps of: preparing a first resin primarily composed of a polylactic acid polymer; and a second resin primarily composed of an aliphatic polyester resin or an aliphatic-aromatic copolymer polyester resin (Step 1); and melt-extruding the first resin and the second resin, respectively, and alternately laminating the first resin layer and the second resin layer to obtain a sheet in which two or more different thermoplastic resin layers are alternately laminated (Step 2).

Steps 1 and 2 can be carried out in the same manner as steps 1 and 2 in the method for producing the multilayer biodegradable film.

[Environmentally friendly packaging materials]
According to one embodiment, the present invention can provide an environmentally friendly packaging material including the multilayer biodegradable film.

Specifically, the environmentally friendly packaging material includes a multilayer biodegradable film in which two or more different thermoplastic resin layers are alternately laminated, including a first resin layer whose main component is a polylactic acid-based polymer and a second resin layer whose main component is an aliphatic polyester-based resin or an aliphatic-aromatic copolymer polyester-based resin, and the lumen uniformity (LUI) of the first resin layer, as expressed by Equation 1, may be 0.2 μm or less.

The environmentally friendly packaging material may be in the form of a film that can be used, for example, as general disposable packaging material or food packaging material, or in the form of a fiber that can be used as a woven fabric, knitted fabric, nonwoven fabric, rope, etc., or in the form of a container that can be used as a food packaging container for lunch boxes, etc.

The environmentally friendly packaging material contains a multi-layer biodegradable film that simultaneously possesses excellent uniformity, flexibility, and transparency, as well as low noise levels, providing excellent physical properties and quality. Furthermore, since it is biodegradable and completely decomposes when landfilled, it can provide a packaging material with environmentally friendly properties, allowing it to be used in a variety of fields as a packaging material and demonstrate its excellent properties.

(Example)
The present invention will be described in more detail below with reference to examples. The following examples are merely illustrative of the present invention, and the scope of the present invention is not limited thereto.

Example 1: Production of a multilayer biodegradable film
The resin used for the first resin layer was a polylactic acid resin (Nature Works LLC, 4032D) with a D-lactide content of approximately 1.4% and a melt viscosity of approximately 8770 poise at approximately 210°C, and the resin used for the second resin layer was a polybutylene adipate terephthalate (PBAT) resin (XINJIANG BLUE RIDGE TUNHE POLYESTER Co., Ltd.), an aliphatic-aromatic copolymer polyester resin with a melt viscosity of approximately 6259 poise at 210°C and an aliphatic component content of 50 mol% in the acid component.

The resin for the first resin layer was dried in a dehumidifying dryer at approximately 80°C for 6 hours, and the resin for the second resin layer was dried in a dehumidifying dryer at approximately 80°C for 2 hours to remove moisture. After that, using two extruders and a multi-layer feed block in which two layers are alternately stacked, the resin for the first resin layer was melt-extruded in an extruder with a temperature of 210°C, and the resin for the second resin layer was melt-extruded in an extruder with a temperature of 210°C.

In the multilayer feed block, the first resin layer was branched into 15 layers and the second resin layer into 14 layers. These branched first and second resin layers were then passed through a 780 mm die so that they were alternately laminated, and then pressed against a cooling roll cooled to approximately 21°C to obtain an unstretched multilayer biodegradable sheet with 29 layers. In this case, the first resin layer was placed as the outermost layer on both the top and bottom surfaces, and the total thickness of the outermost layers on both surfaces was 30% of the total thickness.

The unstretched multilayer biodegradable sheet obtained in this manner was stretched 3.0 times in the longitudinal direction at approximately 65°C and 3.9 times in the transverse direction at 120°C, and then heat-set at 150°C to impart a relaxation rate of 1%, producing a multilayer biodegradable film with a total thickness of 20 μm, 29 layers, and a first resin layer uniformity (LUI) of approximately 0.077 μm.

Example 2: Production of multilayer biodegradable film
A multilayer biodegradable film having 43 layers and a total thickness of 25 μm was prepared in the same manner as in Example 1, except that the thicknesses of the first and second resin layers, the total number of layers of the film, the sum of the thicknesses of the outermost layers on both sides, and the uniformity (LUI) of the first resin layer were adjusted as shown in Table 1.

Example 3: Production of multilayer biodegradable film
A multilayer biodegradable film having 57 layers and a total thickness of 20 μm was prepared in the same manner as in Example 1, except that the thicknesses of the first and second resin layers, the total number of layers of the film, the sum of the thicknesses of the outermost layers on both sides, and the uniformity (LUI) of the first resin layer were adjusted as shown in Table 1.

(Comparative Example 1: Production of a monolayer biodegradable film)
The same polylactic acid resin as in Example 1 was used as the resin for the first resin layer, and this was dried in a dehumidifying dryer at approximately 80°C for 6 hours to remove moisture. The resin for the first resin layer from which the moisture had been removed was melt-extruded using an extruder at a temperature of 210°C, passed through a 780 mm die, and then brought into close contact with a cooling roll cooled to 20°C to obtain a single-layer unstretched sheet. The single-layer unstretched sheet thus obtained was stretched 3.0 times in the longitudinal direction at 65°C and 3.8 times in the transverse direction at 120°C, and then heat-set at 120°C to impart a relaxation rate of 1% to produce a single-layer film with a thickness of 20 μm.

(Comparative Example 2: Production of a monolayer biodegradable film)
The resins for the first resin layer and the second resin layer used in Example 1 were hand-mixed in a weight ratio of 80:20, and then blended in a 45-mm twin-screw extruder at 200° C. This was dried in a dehumidifying dryer at about 60° C. for 8 hours, and then melt-extruded at 200° C. to produce a single-layer film with a thickness of 30 μm.

(Comparative Example 3: Production of multilayer biodegradable film)
As shown in Table 1, the resin for the first resin layer was melt-extruded using an extruder at a temperature of 210°C, and the resin for the second resin layer was melt-extruded using an extruder at a temperature of 180°C. The same procedure as in Example 1 was repeated, except that the thickness of the first resin layer and the second resin layer, the total thickness of the outermost layers on both sides, and the uniformity (LUI) of the first resin layer were adjusted, to produce a multilayer biodegradable film having 29 layers and a total thickness of 20 μm.

(Comparative Example 4: Production of multilayer biodegradable film)
As shown in Table 1, the resin for the first resin layer was melt-extruded using an extruder at a temperature of 210°C, and the resin for the second resin layer was melt-extruded using an extruder at a temperature of 140°C. The same procedure as in Example 2 was carried out, except that the thicknesses of the first and second resin layers, the total thickness of the outermost layers on both sides, and the uniformity (LUI) of the first resin layer were adjusted, to produce a 43-layer multilayer biodegradable film with a total thickness of 25 μm.

(Comparative Example 5: Production of multilayer biodegradable film)
As shown in Table 1, the resin for the first resin layer was melt-extruded using an extruder at a temperature of 210°C, and the resin for the second resin layer was melt-extruded using an extruder at a temperature of 190°C. The same procedure as in Example 3 was carried out, except that the thicknesses of the first and second resin layers, the total thickness of the outermost layers on both sides, and the uniformity (LUI) of the first resin layer were adjusted, to produce a multilayer biodegradable film having 57 layers and a total thickness of 20 μm.

The properties of each layer and process conditions for the single-layer and multi-layer biodegradable films prepared in Examples 1 to 3 and Comparative Examples 1 to 5 are summarized in Table 1 below.

(Evaluation example)
The properties and performance of the single-layer or multi-layer biodegradable films prepared in Examples 1 to 3 and Comparative Examples 1 to 5 were measured and evaluated by the following methods. The results are shown in Table 2 below.

(Evaluation Example 1: Thickness)
The thickness relative to the total width of the single-layer or multi-layer biodegradable films prepared in the Examples and Comparative Examples was measured.

The thickness of the multilayer biodegradable film was measured using an MFC-101 (NIKON Corporation) by dividing the 500 mm film width into 10 intervals and taking the average thickness.

(Evaluation Example 2: Uniformity (LUI))

<Uniformity of the first resin layer of the multilayer biodegradable film>
In the multilayer biodegradable films of Examples 1 to 3 and Comparative Examples 3 to 5, the uniformity (LUI) of the first resin layer was measured.

Specifically, referring to Figures 1 and 3, the center point in the width (W) direction of the multilayer biodegradable films produced in Examples 1 to 3 and Comparative Examples 3 to 5 was designated "C," a point 50 mm away from one end in the width direction (W1) was designated "N," and a point 50 mm away from the other end in the width direction (W2) was designated "S." The multilayer biodegradable films were then cut in the thickness direction (A-A'), and the thickness of each individual layer of the laminated first resin layer was measured at each cut cross section using a field emission scanning electron microscope (FE-SEM) (JSM-6701F, JEOL).

Referring to FIG. 3(b), the thickness of each individual layer of the laminated first resin layer was measured at each position, i.e., at point N ( _tN1 , _tN2 , _tN3 , ...), at point C ( _tc1 , _tC2 , _tC3 , ...), and at point S ( _tS1 , _tS2 , _tS3 , ...). From the thicknesses of each individual layer of the laminated first resin layer at each position, the maximum thicknesses tmax _{, N} , _{tmax, C} , tmax _{, S} , and the minimum thicknesses tmin _{, N} , tmin _{, C} , _{tmin, S} , were calculated, and the uniformity (LUI) expressed by the following equation 1 was measured using each of these values.

The maximum and minimum thicknesses of the individual layers of the laminated first resin layer were determined from the positional thicknesses of the individual layers, and these values were used to measure the uniformity (LUI) of the first resin layer, which is expressed by the following formula 1.

<Formula 1>
In the formula 1,
When the thickness of each layer, excluding the outermost layers on both sides of the laminated first resin layer, was measured using a field emission scanning electron microscope (FE-SEM) in a cross section of the multilayer biodegradable film having a width of 500 mm and a thickness of 20 μm to 25 μm, cut in the thickness direction at a point (N) 50 mm away from one end in the width direction, a point (S) 50 mm away from the other end in the width direction, and a center point (C) in the width direction,
The t _max,N is the maximum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point N of the multilayer biodegradable film;
t _min,N is the minimum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point N of the multilayer biodegradable film;
The t _{max, S} is the maximum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point S of the multilayer biodegradable film;
tmin _,S is the minimum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point S of the multilayer biodegradable film;
The tmax _{, C} is the maximum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point C of the multilayer biodegradable film;
The t _min,C is the minimum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point C of the multilayer biodegradable film.

The individual layers of the first resin layer refer to each layer of the first resin layer excluding the outermost layers on both sides (110' in Figures 1 and 3). Therefore, the maximum and minimum thicknesses of the individual layers 110 of the first resin layer refer to the maximum and minimum thicknesses of the individual layers 110 of the first resin layer excluding the outermost layers 110' on both sides.

<Uniformity of unstretched sheet>
Using the same method as for the uniformity (LUI) of the first resin layer of the multilayer biodegradable film, the maximum and minimum thicknesses of the individual layers of the laminated first resin layer were determined for the multilayer biodegradable sheets (unstretched sheets) of Examples 1 to 3 and Comparative Examples 3 to 5, and these values were used to measure the uniformity (LUI _s ) of the first resin layer, expressed by the following formula 1-1.
<Formula 1-1>
In the formula 1-1,
When the thickness of each of the laminated first resin layers was measured using a field emission scanning electron microscope (FE-SEM) at a cross section of the multilayer biodegradable sheet having a width of 650 mm and a thickness of 300 μm, the cross section was cut in the thickness direction at a point (N) 50 mm away from one end in the width direction, a point (S) 50 mm away from the other end in the width direction, and a center point (C) in the width direction,
The tmax _{, N1} , is the maximum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer, measured at point N of the multilayer biodegradable sheet;
tmin _{, N1} is the minimum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point N of the multilayer biodegradable sheet;
The tmax _{and S1} are the maximum thicknesses of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point S of the multilayer biodegradable sheet;
tmin _{, S1} is the minimum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point S of the multilayer biodegradable sheet;
The tmax _{, C1} , is the maximum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer, measured at point C of the multilayer biodegradable sheet;
The t _{min C1} is the minimum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point C of the multilayer biodegradable sheet.

The individual layers of the first resin layer refer to each layer of the first resin layer excluding the outermost layers on both sides (110' in Figures 1 and 3). Therefore, the maximum and minimum thicknesses of the individual layers 110 of the first resin layer refer to the maximum and minimum thicknesses of the individual layers 110 excluding the outermost layers 110' on both sides of the first resin layer.

(Evaluation Example 3: Haze (HZ))
Measurement was carried out using a haze meter (model name: SEP-H) manufactured by Nippon Seimitsu Kogaku Co., Ltd., in accordance with the ASTM D1003 standard.

(Evaluation Example 4: Young's Modulus (YM))
Test specimens were prepared from the monolayer or multilayer biodegradable films prepared in the examples and comparative examples in accordance with ASTM D882, cut to a length of 150 mm and a width of 15 mm, and then attached to a chuck with a 50 mm gap. The test specimens were subjected to a tensile test using an Instron 5566A tensile tester at a tensile speed of 200 mm/min, and the linear gradient from the start of measurement to the point where an elongation of 3% was reached was measured as Young's modulus (kgf/ ^mm2 ).

(Evaluation Example 5: Noise Level (N _AVG ))
The monolayer or multilayer biodegradable films produced in the Examples and Comparative Examples were cut into A4 size pieces of 210 mm x 297 mm in a polycarbonate box measuring 650 (W) mm x 450 (D) mm x 500 (H) mm. The multilayer biodegradable films were positioned 30 cm away from a digital noise analyzer (Cirrus Research Plc, model name: CR-162C), and both ends of the film were gripped with a jig and repeatedly twisted back and forth at a speed of 30 times per minute. The noise intensity was measured five times when noise was produced for 5 seconds or more, and the noise intensity was calculated.

(Evaluation Example 6: Flexible Noise Complexity (FNC))
Using the values of YM and N _AVG measured in Evaluation Examples 4 and 5, the flexible noise composite (FNC) was calculated using the following equation 3.

<Formula 3>
In the formula 3,
The YM and N _AVG are unit-free values measured on the multilayer biodegradable film test piece,
The YM is Young's modulus, which is a linear gradient value from the measurement start point to the point where an elongation of 3% is reached, after preparing a multilayer biodegradable film test piece in accordance with ASTM D882, cutting it into a length of 150 mm and a width of 15 mm, and attaching it so that the gap between chucks is 50 mm and testing it at a tensile speed of 200 mm/min,
The N _AVG is the average noise intensity (dB) calculated by measuring five times the noise intensity while a multilayer biodegradable film cut to A4 size of 210 mm x 297 mm was placed 30 cm away from a digital noise analyzer in a polycarbonate box of 650 (W) mm x 450 (D) mm x 500 (H) mm, and the film was gripped at both ends with a jig and repeatedly twisted back and forth at a speed of 30 times per minute, producing noise for at least 5 seconds.

(Evaluation Example 7: Appearance Noise Quality Composite Index (QCI))
Using the values of LUI, HZ, and N _AVG measured in Evaluation Examples 2, 3, and 5, the appearance noise quality composite index (QCI) was calculated as shown in the following equation 4.

<Formula 4>
In the formula 4,
The HZ, LUI, and N _AVG are unit-free values measured on the multilayer biodegradable film specimen,
HZ is the haze (%) of the multilayer biodegradable film,
The LUI is the uniformity expressed by the formula 1,
The N _AVG is the average noise intensity (dB) calculated by measuring five times the noise intensity while a multilayer biodegradable film cut to A4 size of 210 mm x 297 mm was placed 30 cm away from a digital noise analyzer in a polycarbonate box of 650 (W) mm x 450 (D) mm x 500 (H) mm, and the film was gripped at both ends with a jig and repeatedly twisted back and forth at a speed of 30 times per minute, producing noise for at least 5 seconds.

As can be seen from Table 2, the multilayer biodegradable sheets and films of Examples 1 to 3 according to the present invention had significantly better uniformity of the first resin layer than the single-layer or multilayer biodegradable sheets and films of the comparative examples, and were also superior in all physical properties, including flexibility, noise level, and transparency.

Specifically, with regard to the uniformity of the first resin layer, the uniformity (LUI _s ) of the first resin layer of the multilayer biodegradable sheets of Examples 1 to 3 was 1.12 to 1.91, and the uniformity (LUI) of the first resin layer of the multilayer biodegradable films was 0.077 to 0.122. This shows that the uniformity of the first resin layer was significantly improved compared to the multilayer biodegradable sheets and films of Comparative Examples 3 to 5, whose LUI _s and LUI were 2.3 or more and 0.221 or more, respectively.

In terms of flexibility and noise level, the multilayer biodegradable films of Examples 1 to 3 had a Young's modulus of 221 kgf/ ^mm2 to 233 kgf/ ^mm2 and a noise level of 79.5 dB or less, whereas the single-layer biodegradable film of Comparative Example 1 had a Young's modulus of 384 kgf/ ^mm2 and a noise level of 88.3 dB, demonstrating reduced flexibility and a significantly higher noise level. Furthermore, when compared with the multilayer biodegradable films of Examples 1 to 3 having the same number of layers and thickness, the multilayer biodegradable films of Comparative Examples 3 to 5 were confirmed to have reduced flexibility and a significantly higher noise level.

On the other hand, with regard to transparency, the multilayer biodegradable films of Examples 1 to 3 all had a haze of 3% or less, demonstrating excellent transparency, while the single-layer or multilayer biodegradable films of Comparative Examples 1 to 3 and 5 had a haze exceeding approximately 3%, and in particular, the single-layer biodegradable film of Comparative Example 2 had a haze of 25%, indicating a significant decrease in transparency.

Therefore, the multilayer biodegradable sheets and films of Examples 1 to 3 of the present invention not only have excellent biodegradability, but also have very good uniformity in the first resin layer, and the final film has excellent uniformity, flexibility, noise level, transparency, and appearance properties, so it can be confirmed that they can be used as environmentally friendly products in a variety of applications, including packaging applications such as food packaging materials.

100: Multilayer biodegradable film 110: First resin layer (individual layer)
110': outermost layer on both sides 120: second resin layer (individual layer)
N: A point 50 mm away from one end of the multilayer biodegradable film in the width direction. C: The center point of the multilayer biodegradable film in the width direction. S: A point 50 mm away from the other end of the multilayer biodegradable film in the width direction. W: Width. W1: Width away from one end of the multilayer biodegradable film. W2: Width away from the other end of the multilayer biodegradable film. L: Length. A-A': Incision line.

Claims (11)

The thermoplastic resin layer is formed by alternately laminating five or more layers of two different types of thermoplastic resins, the first resin layer being mainly composed of a polylactic acid polymer and the second resin layer being mainly composed of an aliphatic polyester resin or an aliphatic- aromatic copolymer polyester resin,
A multilayer biodegradable film, wherein the first resin layer has a uniformity (LUI) represented by the following formula 1 of 0.2 μm or less:
<Formula 1>
In the formula 1,
When the thickness of each layer, excluding the outermost layers on both sides of the laminated first resin layer, was measured using a field emission scanning electron microscope (FE-SEM) in a cross section of the multilayer biodegradable film having a width of 500 mm and a thickness of 20 μm to 25 μm, cut in the thickness direction at a point (N) 50 mm away from one end in the width direction, a point (S) 50 mm away from the other end in the width direction, and a center point (C) in the width direction,
The t _max,N is the maximum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point N of the multilayer biodegradable film;
t _min,N is the minimum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point N of the multilayer biodegradable film;
The t _{max, S} is the maximum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point S of the multilayer biodegradable film;
tmin _,S is the minimum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point S of the multilayer biodegradable film;
The tmax _{, C} is the maximum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point C of the multilayer biodegradable film;
The t _min,C is the minimum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point C of the multilayer biodegradable film. The multilayer biodegradable film according to claim 1, wherein the difference in uniformity (Δt _N,S ) between both ends in the width direction of the multilayer biodegradable film, represented by the following formula 2, is 0.06 μm or less:
<Formula 2>
In the formula 2,
The t _{max,N 1} , t _{min,N 2} , t _max,S 1 and t _min,S are as defined in claim 1 . The multilayer biodegradable film according to claim 1, wherein the flexibility noise composite (FNC) represented by the following formula 3 is 20 or less:
<Formula 3>
In the formula 3,
The YM and N _AVG are unit-free values measured on the multilayer biodegradable film test piece,
The YM is Young's modulus (kgf/mm2), which is a linear gradient value from the measurement start point to the point where an elongation rate of 3% is reached, after preparing a multilayer biodegradable film test piece according to ASTM D882, cutting it into a length of 150 mm and a width of 15 mm, and attaching it so that the gap between chucks is 50 mm and testing it at a pulling speed of ²⁰⁰ mm/min.
The N _AVG is the average noise intensity (dB) calculated by measuring five times the noise intensity while a multilayer biodegradable film cut to A4 size of 210 mm x 297 mm was placed 30 cm away from a digital noise analyzer in a polycarbonate box of 650 (W) mm x 450 (D) mm x 500 (H) mm, and the film was gripped at both ends with a jig and repeatedly twisted back and forth at a speed of 30 times per minute, producing noise for at least 5 seconds. The multilayer biodegradable film according to claim 3, wherein the film has an appearance noise quality composite index (QCI) of 28 or less, represented by the following formula 4:
<Formula 4>
In the formula 4,
The HZ, LUI, and N _AVG are unit-free values measured on the multilayer biodegradable film specimen,
HZ is the haze (%) of the multilayer biodegradable film,
The LUI is as defined in claim 1,
The _{N_AVG} is as defined in claim 3. the outermost layers on both sides of the film are first resin layers,
The multilayer biodegradable film according to claim 1, wherein the total thickness of the outermost layers on both sides is 5% to 40% of the total thickness of the film. The film has 5 or more layers,
The multilayer biodegradable film according to claim 1, wherein the average thickness ratio of the first resin layer and the second resin layer, excluding the outermost layers on both sides, is 1:0.5-2. The multilayer biodegradable film according to claim 1, wherein the aliphatic component content in the acid component of the aliphatic-aromatic copolymer polyester resin is 30 mol% or more. The thermoplastic resin layer is formed by alternately laminating five or more layers of two different types of thermoplastic resins, the first resin layer being mainly composed of a polylactic acid polymer and the second resin layer being mainly composed of an aliphatic polyester resin or an aliphatic- aromatic copolymer polyester resin,
A multilayer biodegradable sheet in which the uniformity (LUI _S ) of the first resin layer represented by the following formula 1-1 is less than 2.3 μm:
<Formula 1-1>
In the formula 1-1,
The multilayer biodegradable sheet has a width of 650 mm and a thickness of 300 μm. When the thickness of each layer, excluding the outermost layers on both sides of the laminated first resin layer, was measured using a field emission scanning electron microscope (FE-SEM) at a cross section cut in the thickness direction at a point (N) 50 mm away from one end in the width direction, a point (S) 50 mm away from the other end in the width direction, and a center point (C) in the width direction,
The tmax _{, N1} , is the maximum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer, measured at point N of the multilayer biodegradable sheet;
tmin _{, N1} is the minimum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point N of the multilayer biodegradable sheet;
The tmax _{and S1} are the maximum thicknesses of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point S of the multilayer biodegradable sheet;
tmin _{, S1} is the minimum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point S of the multilayer biodegradable sheet;
The tmax _{, C1} , is the maximum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer, measured at point C of the multilayer biodegradable sheet;
The t _{min C1} is the minimum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point C of the multilayer biodegradable sheet. A step (step 1) of preparing a first resin mainly composed of a polylactic acid polymer and a second resin mainly composed of an aliphatic polyester resin or an aliphatic-aromatic copolymer polyester resin;
a step (step 2) of melt-extruding the first resin and the second resin, respectively, and alternately laminating first resin layers and second resin layers to obtain a sheet having five or more layers of two different thermoplastic resin layers alternately laminated therein;
and (3) biaxially stretching and heat-setting the laminated sheet to obtain a multilayer biodegradable film.
The multilayer biodegradable film has a uniformity (LUI) of 0.2 μm or less, an interlayer melt viscosity difference (ΔV) at 210° C. of 500 poise or more, and a difference between the melt extrusion temperature of the first resin and the melt extrusion temperature of the second resin of less than 30° C., wherein :
<Formula 1>
<Formula 5>
ΔV=V1-V2
In the formula 1,
When the thickness of each layer, excluding the outermost layers on both sides of the laminated first resin layer, was measured using a field emission scanning electron microscope (FE-SEM) in a cross section of the multilayer biodegradable film having a width of 500 mm and a thickness of 20 μm to 25 μm, cut in the thickness direction at a point (N) 50 mm away from one end in the width direction, a point (S) 50 mm away from the other end in the width direction, and a center point (C) in the width direction,
The t _max,N is the maximum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point N of the multilayer biodegradable film;
t _min,N is the minimum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point N of the multilayer biodegradable film;
The t _{max, S} is the maximum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point S of the multilayer biodegradable film;
tmin _,S is the minimum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point S of the multilayer biodegradable film;
The tmax _{, C} is the maximum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point C of the multilayer biodegradable film;
tmin _,C is the minimum thickness of the individual layers excluding the outermost layers on both sides of the first resin layer measured at point C of the multilayer biodegradable film;
In the formula 5,
V1 is the melt viscosity of the first resin layer,
V2 is the melt viscosity of the second resin layer . The melt extrusion temperature of the first resin is greater than 180°C and less than 250°C,
The method for producing a multilayer biodegradable film according to claim 9, wherein the melt extrusion temperature of the second resin is greater than 180°C and less than 250°C. An environmentally friendly packaging material comprising the multilayer biodegradable film of claim 1.